1. Field of the Invention
The present invention relates to a modem for use in a data transmission system, the modem comprising a data send port for sending data to a remote apparatus, a data receive port for receiving data originating from the remote apparatus, and echo cancelling means. The latter consists of filtering means having adjustable coefficients and phase-shifting means cascaded therewith for supplying filtered and phase-shifted information, originating from the send port to a subtraction means for subtraction from the data received at the receive port. Control means adjusts the filtering means and the phase-shifting means so that said subtraction cancels out any possible echo at the output of the subtraction means arising in transmission between the modem and the remote apparatus.
2. Description of the Related Art
A modem of this kind is described in published European Patent Application No. 0 334 442. Although this provides satisfactory echo cancellation the start-up period is somewhat too long. The start-up period consists of a dialogue between the remote apparatus and the modem, and is a necessary preliminary before commencing transmission of useful data between them. This is described in C.C.I.T.T. standards V32 and V32bis in particular.